The expansive data transmission capabilities of optical fiber technology have made practical the operation of digital telecommunication systems at rates well into the gigabit per second (Gbit/s) range. The advantages to be realized in this technology are apparent, and development of such systems has proceeded on numerous fronts worldwide. Unfortunately, these contemporaneous developments have resulted in a number of independently-devised signal architectures which lack the compatibility necessary for effective global, or even regional, communications networks.
With a view toward establishing and maintaining such compatibility, standards bodies have endorsed basic structures of optical system transmission rates and interfaces, not the least among which are those incorporated into the Synchronous Optical Network (SONET) hierarchy concept. This important advancement operates upon a base level digital signal framing format, namely the Synchronous Transport Signal level 1 (STS-1) frame, which consists of 810 8-bit bytes of data, and which therefore provides a serial bit transmission rate of 51.84 Mbit/s at the basic 8000 per second frame transmission rate.
Under this concept, signal transmissions of higher rate are achieved by interleaving bytes of any desired number of STS-1 frames in a prescribed sequence to form the correspondingly higher signal levels, e.g. STS-3, STS-4, STS-6, . . ., STS-24, etc. The STS-24 signal thus consists of the interleaved bytes of 24 STS-1 signals, and has a resulting transmission rate of 1244 Mbit/s, or 1.244 Gbit/s, i.e. 24 times the rate of the basic 51.84 Mbit/s of the STS-1 signal. For the transmission of such a signal, a multiplexed serial bit stream is assembled by interleaving repeated sequential extractions of one byte from each of the component STS-1 frames. It is necessary, therefore, that the signal receiver reconstruct from this serial bit stream the original base frame, or some frame multiple thereof, in order that the correct substance of the transmitted signal may be recovered.
While with current technology the serial bit stream may be assembled into fundamental 8-bit byte structures, it is essential to the proper recovery of the original SONET frames that the byte assembly be correctly synchronized and the boundaries of each such frame be identified in the bit stream transmission in order that the reconstructed bytes will duplicate each of the bytes which were interleaved to produce that serial transmission signal. The present invention provides method and apparatus to ensure that such proper synchronization and frame identification are established and maintained throughout such a signal transmission.